1. Field of the Invention
The present invention relates to a focusing method for an optical system and an image-taking apparatus. The invention relates, for example, to a focusing method used in any types of non-coaxial optical systems including an image-taking optical system, a projection optical system, and the like, and to an image-taking apparatus adopting this focusing method for the image-taking optical system. Examples of the image-taking optical system include an optical system forming an optical image of a subject on the light-receiving surface of an image sensor in a digital appliance equipped with an image input capability (for example, a cellular phone). Examples of the projection optical system include, for example, an optical system projecting an image of a display element on a screen in a projector.
2. Description of Related Art
In recent years, to achieve downsizing of an image-taking optical system and a projection optical system, a non-coaxial optical system has been under development which bends an optical path by using an eccentric optical surface (for example, a free-form surface formed into a rotationally-asymmetric, curved shape). To put the image-taking optical system and the projection optical system into practice in the non-coaxial optical system, a focusing method is required which is capable of ensuring favorable imaging performance even at different photographing distances or projection distances. Focusing methods for a non-coaxial optical system conventionally proposed include, for example, those proposed in Patent Documents 1 and 2, and the like. In the non-coaxial optical system described in Patent Document 1, an axial incidence principal ray and an axial exit principal ray of a focusing group are parallel to each other, and focusing is achieved by parallel movement of the focusing group in parallel to the direction in which these principal rays are parallel to each other. In the non-coaxial optical system described in Patent Document 2, an axial incidence principal ray and an axial exit principal ray of an entire optical system are parallel to each other, and focusing is achieved by parallel movement of the focusing group in parallel to the direction in which these principal rays are parallel to each other.
Focusing methods for a projection optical system using an eccentric optical lens include those described in Patent Documents 3 to 5. Patent Documents 3 to 5 propose optical systems with little trapezium distortion which satisfy relationship between inclinations of an object surface and an image surface based on “T. Scheimpflug” theory. Although no detailed numerical examples are provided in the description, the configuration is provided for an optical system with a coaxial system lens inclined with respect to the optical axis, and an image with little trapezium distortion is provided by arranging the coaxial system lens so that the T. Scheimpflug theory holds for an inclined object surface. Then focusing is achieved by changing an optical lens, the image surface position, and the object surface position so that the T. Scheimpflug theory holds.
[Patent Document 1]JP-A-H8-292368[Patent Document 2]JP-A-2005-134832[Patent Document 3]JP-A-H5-80418[Patent Document 4]JP-A-H5-113600[Patent Document 5]JP-A-H6-148566
With the focusing methods proposed in Patent Documents 1 and 2, the axial incidence principal ray and axial exit principal ray of the focusing group or the entire system are parallel to each other, and focusing is achieved by the parallel movement of the focusing group or the entire system in parallel to the direction in which these principal rays are parallel to each other. In achieving the downsizing and slimming-down, optical path bending in the non-coaxial optical system is more effective than optical path bending in a coaxial optical system. That is, orienting an axial incidence principal ray and axial exit principal ray of an optical system or an optical element in parallel reduces effect of downsizing and slimming-down achieved by optical path bending, thus resulting in failure to make full use of an advantage provided by the non-coaxial optical system.
While having the advantage described above, the non-coaxial optical system, risk fluctuation in the angle of incidence of a principal ray located at the image center (that is, axial principal ray) on the image surface during focusing. A great change from vertical incidence of the axial principal ray with respect on the image surface in particular causes non-uniformity in the amount of light at this position, which provides asymmetrical brightness distribution, thereby causing non-uniformity in the brightness of an image. Consequently, the non-uniformity in the brightness of an image results and also this non-uniformity changes following focusing in particular; thus leading to failure to provide a clear image, which is not practical.
The focusing methods proposed in Patent Documents 3 to 5 are difficult to be applied to a more eccentric non-coaxial optical system using an eccentric reflective surface, because the T. Scheimpflug's theory does not apply to such a non-coaxial optical system. Application of the T. Scheimpflug's theory required an optical axis of one straight line, which usually does not exist in the non-coaxial optical system. Therefore, in application of the focusing methods proposed in Patent Documents 3 to 5 to focusing, it is impossible to perform such focusing as is performed by an aggressive non-coaxial optical system in which the optical path is bent.